Thursday, September 13, 2012

Electric Kiln Ideas

Downdraft ventilation protects connections from corrosion

I have a top-loading Cone Art electric kiln that is nearly 20 years old. It is equipped with a Perfect Fire programmable controller.

In my former studio, I used a downdraft ventilation system. The kiln was fired zillions of times to cone 9 in that studio. The elements had to be changed a couple of times when firing times got unacceptably long. However, during that period, the inside of the control box remained pretty clean and I never experienced failure of an electrical connection caused by corrosion.

In my current studio, I got lazy. Since the kiln is in a small room with windows that permit a cross-draft, instead of hooking up a downdraft vent system, I've been venting the room by opening the windows and putting a fan in one of the windows. After less than 30 firings, two elements failed due to corrosion at the connectors. Rust seems to collect on the floor, falling from the inner surface of the stainless steel kiln sheath. There is corrosive dust coating everything inside the control box.

Here is what I think must be going on.... When you have a downdraft ventilation system, you are sucking gases at a slow rate from inside the kiln and expelling them outside the building. Anyone who has used a vent like this with metal ducts soon learns that kiln fumes are highly corrosive to metal. Where do these corrosive fumes go if you are not sucking them out through your duct-work? Some fumes come out the top. Some penetrate right through the porous brick and corrode the inside surface of the metal sheath of the kiln. And some enter the control box and corrode the connections.

When you use a downdraft vent system, the gases that are sucked out of the kiln have to be replaced by fresh room air sucked into the kiln. Assuming your peep holes are closed most of the time, the only other way for make-up air to enter the kiln is from the top and through numerous holes at the back of the control box. At the back of the control box, there are two holes for each element and another hole for the thermocouple. When your downdraft vent fan is running, there is a small flow of clean room air that passes through the control box and into the kiln. This inward direction of air flow prevents corrosive fumes from entering the control box.

After I clean the corrosive dust out of the control box and clean up my wires and connectors, and rewire the kiln (see next section), I will install a downdraft vent.

Simplify the wiring

I haven't worked on many different electric kilns. My guess is that there probably are a lot of electric kilns like mine that have goofy wiring. The goofiness probably resulted from the kiln originally being designed to work with switches and later retrofitted with a programmable controller.

The programmable controller on my kiln operates 4 different relays. When the controller sends a small, 12-volt current through one of the relays, a 2-pole switch in the relay closes a circuit that passes a 240-volt current of up to 25 amps. This amp rating is enough to power two 10-amp kiln elements.  Each of my 4 relays could simply be wired directly to two kiln elements. That would be simple and logical and make it easy to track down the cause of any elements failing to work properly.

What I actually have is a goofy setup. There are 3 needless switches. The power coming from relay #3 powers elements 5 and 6 directly. Power from relay #4 powers element 7 directly but sends power to element 8 through the bottom switch. Relay #1 sends power to elements 1 and 2 through the top switch. Relay #2 sends power to elements 3 and 4 through the middle switch.

My middle switch got so corroded that it no longer passes current. My first thought was that I need to replace it. I found that I could buy similar infinite switches on ebay for around $10 or from a kiln supplier for $35 - $45, but as I studied the wiring and how these switches work, ... I realized that current simply passes through them. The programmable controller regulates the kiln. The switches regulate nothing.

These switches (infinite switches) are kept at their "high" setting at all times, meaning that current simply passes through the switch and is never interrupted by the switch. The switches have nothing at all to do with regulating power to the kiln elements. There is no reason why power should not pass directly from the relays to the elements, bypassing the switches.

Here is what I plan to do.... I will rewire the kiln such that each of the 4 relays will power 2 elements directly. I will remove the switches. If two elements powered by the same relay get stuck on or off, I will immediately know which relay is not working properly.

Smoking in an electric kiln

Last winter I enjoyed making small pieces, decorating with terra sig, and firing and smoking pots in a wood stove. I'm now thinking of doing similar but larger pieces in an electric kiln.

I know that there are a lot of people who have tried enclosing their pots with combustibles within aluminum foil "saggars". Others have used ceramic saggars. Others have pulled pots with tongs and placed them into closed containers with combustibles. I think there is an assumption behind all these methods that smoke is bad for electric kiln elements. 

However, I've seen a video showing Michael Wisner using combustibles right on the shelf next to a pot in an electric kiln. In other words, the whole kiln can be the saggar. How bad is that for the elements?

I'm thinking that if  you can fire a kiln full of corrosive fumes to cone 9 more than 100 times before you need to replace the elements, electric kiln elements must be pretty darn tough. You can do your smoking at a fairly low temperature - say 500 degrees F.  It's hard for me to imagine how low temperature smoke could be more harmful than those high temperature corrosive fumes in a cone 9 firing.

Anyway, I've had experience coiling my own elements and replacing kiln elements ... and it's not all that expensive or difficult.

What I'm think of doing is this.... I will fire earthenware pieces decorated with polished terra sig to cone 05. Then, I will do a a smoke firing in the kiln to 500 degrees for just a few minutes with combustibles right in the kiln. I think I will be able to do this many, many times on one set of elements. I'll let you know....

Thursday, January 12, 2012

Woodstove raku

It's winter in Keene, NY.  Winters here are long.  Temperatures can get down to -20 F or less.

This is my second winter of heating this house with wood.  I like the physical and repetitive and traditional qualities of heating with wood.  You cut it, split it, stack it, bring it into the house, stack it again, feed it into the stove, clean out the ashes, sweep up the bark and ashes that collect around the stove, find uses for the ash, keep the chimney clear.

I've been firing and smoking pots right in the woodstove, pulling them out when they seem "done".  It's raku.

The results may not be as functional as stoneware - but the process is good for small sculpture. It's fast and easy, invites experimentation. Smoking emphasizes marks and structure. There is the excitement of unplanned firing effects.  Multiple firings - no problem. 

This is fun.  It's play.  Science would spoil it's best features.  But, I do intend to put a cone pack in there (one of these days) to see just how hot it gets.  I will report my results.

This is the most economical of all ways to fire pots.  I am already burning wood to heat my house; there is no additional cost for firing little pots buried down in the ashes in the woodstove.

Also, this way of firing uses a renewable resource for fuel and adds no more carbon to the environment than the amount produced by heating my home.

High fire adds technical difficulties and uses a whole lot more fuel.  If the work is non-utilitarian, high fire does not necessarily contribute important visual qualities that cannot be achieved in low fire.  Low fire is easier on the wallet, easier on the environment, and capable of producing work with powerful visual presence.

To smoke these pots, I put them into a covered tin can along with paper or sawdust ... and stick the can with pot and combustibles back into the woodstove for 15 minutes or so.  I've been pulling the can back out if smoke stops coming from the can.  If the pot didn't get enough smoke, I put it back with more combustibles for a while longer. The smoke that escapes the can goes right up the chimney ... or you can see the wood gas burning where it escapes the can and finds more oxygen.

After the firing, the pot is likely to have some crusty ash on the surface.  This is easily scrubbed off using an abrasive scrubber and scrubbing compound (e.g. Comet).


In using colored slip on this work, I found that the slip I usually use for high-fire work sticks well to the earthenware pot.  However, because it remains porous like the body, it will take up the carbon from the smoke just as readily as the white earthenware body of the pot ... meaning the slip and the body both turn black and you lose the color.  In order to keep the color, I found that adding borax to the slip will cause it to become a low-temperature glass in the initial part of the firing (in the coals, not yet in the smoking can).  Then when I expose the pot to smoke, the glassy colored slip seals out the smoke ... and the color shines through the surrounding black.  Cool beans.


I'm guessing that I could paint on a solution of borax to seal the surface of the claybody or porous slip and cause these borax-painted areas to resist the smoke.  This would be the reverse of the effect of sealing in carbon by painting a high fire carbon trap glaze with soda ash; I would be using the borax to seal the carbon out, not in. The soda ash causes the glaze to get glassy after the carbon is already absorbed and keeps the carbon from getting burned out as the temperature climbs.  In this raku process, the borax causes the glaze to get glassy before the carbon is absorbed and keeps it from getting absorbed during the smoking.

After a while I will probably want to experiment with terra sig and burnishing.

I can envision an ever-expanding low-fire career unfolding.

That's all for now.  It's snowing outside.  I have to stack the firewood that I split yesterday before it gets buried under the snow.  The wood will be stacked outside to dry for a year - to be fuel for next winter's heating/firing season.

Saturday, January 14, 2012 1:08:20 PM

I got the wood stacked and then started up the snow-blower ... only to discover that the snow-blower wouldn't move.  After tearing the machine apart (something I've never done before), I found a broken weld in the mechanism that thrusts the drive plate against the friction wheel.  (You could use a mechanism like this to make a variable speed potter's wheel using a constant-speed motor.  I should be possible make a good potter's wheel from an old snow-blower transmission.)

I discovered online that you can weld steel using 2 or 3 car batteries connected in series (link).  I was tempted to explore these new heights of resourcefulness, but I decided that my time would be better spent trying to get the help of someone with a welding machine and experience.  I brought the broken part to a local auto repair garage ... and the guy did a very nice job of welding the two pieces back together - better than the original. It took him less than 10 minutes.  He charged me $10.  

Now I've got to put the blower back together and blow the snow.  Then I'll need to bring in firewood.  Then I need to go buy some food.  (There is nothing in the refrigerator but a single bottle of beer, a partial thing of celery, and 7 eggs.  However, there is enough rice, pasta, and sardines to survive for a week or more.  And there is a turkey in the feezer.)

I wish I didn't have so many different non-clay jobs to do and could just do clay.  But then, I would have a different life, be a different person, and make different pots.  Writing this blog is actually helping me think through and do all my non-clay tasks ... without getting discouraged or side-tracked.

The blog-writing process helps me remember and develop the real Bill.  In part, this is because I am imagining that there is at least one reader out there who would like to know who I really am.

Note: Yesterday, I found my old Nikon 55 mm f/3.5 macro lens which will work fine on my Nikon D200 digital camera for photographing these little woodstove pots.  The macro lens will let me get close enough to fill the frame.

Sunday, January 15, 2012 12:48:20 PM

Of course, I overestimated my abilities.  It took me until late last evening to get the snowblower together.  There was not really a whole lot of snow to blow ... and much of it was wet and glommed together.  I went to bed without trying to blow, and woke up with the temp outside of -5 F and the driveway iced up, leaving mostly ice and little white stuff that would go through a snow-blower.  I threw wood ashes on the driveway hoping it would add some traction.

I still have to bring in wood and shop for food and make my cone pack.

Sunday, January 15, 2012 5:44:56 PM

Wood in - made a big double-row near the woodstove.

Cone pack drying.  I will be testing from cone 010 to cone 03.


I was going out the door to go grocery shopping when I heard a scratching sound.  It was coming from a plastic bucket near the door in which I keep salt for melting the ice on the walk outside the door.   A little mouse had fallen into the bucket and was struggling to get out - very cute, but not a creature that I wanted to share my house with.

What to do?  I tried to think of a humane way to get rid of the mouse.  If I threw it outside it would slowly freeze to death.  Drown it in the toilet? - ughhh.  Try to clobber it with a stick or stomp on it? - I might miss or just wound it ... and cause it to suffer.   Then it occurred to me: Why don't I toss it into the woodstove where the heat should kill it almost instantly?  That seemed pretty humane ... so I opened the stove door and flipped the mouse in.  The mouse landed on a recently-added piece of wood that was not yet burning ... and immediately ran out of the stove and hid under the wood I just stacked next to the stove.  Curious, brave, lucky little creature lives another day (just like me).  

I'm not sure if there is a lesson to be learned in this experience. Does it mean that I am indecisive, unable to carry out tasks about which I am ambivalent, that I have a cruel streak, that there is a higher power looking out for the mouse and/or me, that my best intentions were ill-conceived and perhaps immoral, or just that life is full of surprises?  Maybe all of those.  I dunno.

Monday, January 16, 2012 12:34:07 AM


I returned from Lake Placid with groceries. The temperature was about -5 F.  In trying to carry too many bags at once while wearing heavy mittens, I dropped one of the bags - which turned out to be the one containing two dozen eggs.  (Apparently, this is the fate of those who throw mice into the woodstove.)

Only 6 of the 24 eggs were broken (lucky me, cursed only 25%).  I decided to have a rather large omelet for dinner.

Temperature inside the woodstove; frozen pipes

After dinner the cone pack seemed dry enough to go into the woodstove.  I stuck it in, being careful to keep it level and very close to the coals. It did not blow up (proof of dryness).

I thought I'd take a shower while the cone pack was absorbing heat.  It turned out that the hot water pipe to the shower was frozen (the apparent fate of those who throw mice into the woodstove and drop eggs).  So, I didn't take a shower.

Interesting but in no way helpful to me: It has been known for centuries that hot water can freeze before cold water. I found explanations for this counter-intuitive phenomenon online (link, link, link). I now have to deal with plumbing issues before the other non-clay jobs I have to do ... which leaves me even less time and energy to do clay.

Annoyed and stubbornly ignoring the frozen pipes, I sat by the woodstove, noodling around on my harmonica and watching the cone pack through the little glass window on the woodstove door.

After a while, the results became clear.  The first cone, which was cone 010, corresponding to approximately 1620 F, was the only cone that actually bent.  The second cone (cone 08) had only the slightest hint of softening.

Speculation ... Intellectualization

If you wanted to be get a highly vitrified clay body from a woodstove firing, the stuff cone 07 is made of might do it.  However, actual cones are formed in a mold with materials that are unlikely to be plastic enough to use for hand-building.

Vitrification might not be all that desirable, anyway. A more vitrified body would take up smoke less readily.  Also, it would probably be more vulnerable to cracking from uneven heating and cooling.

It might be interesting to try to develop a vitrified, low-expansion woodstove body fluxed with lithium which would expand and contract less.  Hmnn....

The clay I used to make the cone pack was stoneware.  The stoneware clay got hard in the woodstove just like the earthenware clay.  Either clay body withstood the shock of being taken from room temperature, placed on hot coals, and then pulled back out into room temperature.  Either clay body is durable enough to be washed with water and scrubbed with a brush and cleanser.  This work does not need to hold food or tolerate freeze-thaw cycles outdoors ... so who cares how vitrified the clay is?

Real Bill says

Art is purposeful play.  It's a performance fixed in time.  The process is more important than any talk about it.  Too much intellectualization undermines the work's freshness and spirit, ... the visual evidence of spontaneity that gives a piece of work its energy.  Art needs science ... but science is not art.

Do I want to do art? Yes.

Do I want to do science?  Nahh. Science is no fun. It stresses me out. It's like going into medicine when I should have become a potter.  I'll do science if I run into a snag.  I should keep outside of the science box as much as possible:  I'll  be more true to myself, happier, more imaginative, more productive.

Tuesday, January 17, 2012 1:04:19 AM

I got my frozen pipes thawed out.  Lucky me they didn't break.  And now I can take a shower. What a luxury!

It warmed up to +30 F (balmy) ... so I have time to figure out how to insulate the pipes better and keep cold drafts out of the crawl space. I will crawl around looking for gaps between the stone foundation and the sill plate to fill with insulating foam.  I will look for cold drafts.  I can cover a lot of the cold stone foundation with fiberglass.  I can put some styrofoam behind the little door to the crawl space.  I can put a little heater down there or wrap the pipe with a heating tape.

My next big project (after I finish doing the sheetrock, painting, and floor in the downstairs bathroom) will be to replace all the corroded water supply lines in the house.  I will use PEX which is elastic enough to withstand freezing without breaking.  From what I've read, this job doesn't seem too hard.  However, I've never done this sort of work before, so I expect it to be stressful and full of surprises and trips to the hardware store.

I will be getting another load of green firewood sometime soon.  My wood guy, Jeff, said it would be today but that didn't happen.  The wood will need to be split and stacked when it comes.

In between all this non-clay stuff, I will keep making, firing, and photographing pots.  I will share images of this work with you ... along with snippets of what my life is like.

Thursday, February 02, 2012 9:26:45 AM

Wow!  I see that my last entry was about 2 weeks ago.  The gap illustrates the amount of non-clay stuff filling up my time and keeping me from playing with clay.  Ah, well.  Here I am, back again.

I made some larger pieces with thicker walls ... and quickly discovered that the clay body I have been experimenting with has too high a maturation temperature to be fired in a woodstove.  These larger, thicker-walled pieces developed cracks ... and the shards seem weak, under-fired, wimpy. The body I have been experimenting with is a white earthenware that matures at cone 05.  It is probably a "talc body".  I know that my woodstove doesn't get anywhere near 05.

I think the thing that bugs me the most about discovering that this clay fires weak is that when I examine the inside of the pot, the outside seems phony.  The outside appearance is bold, rugged ... but the clay is wimpy.

Working with clay helps me know myself.  My life has been an effort to be authentic and to do good stuff ... for fear that without this effort, my existence would mean nothing. I don't like to make something with an outwardly bold appearance that is actually wimpy on the inside. It's not the way I want to be and not the way I want my pots to be. Yet, when I step back from this concern, wonder at the anxiety I feel over the discovery of inner weakness, I am also reminded that in the grand scheme of things, my existence really is pretty inconsequential regardless of my hopes and grand plans.  I can feel humble about this without feeling ashamed or worthless.

Anyway, I got some recipes for claybodies that mature in the range of cone 010 (which is the temperature I measured in the woodstove).  I will do some tests on different bodies and see if I can come up with something.

I have also started experimenting with terrasig and patinas.  I am gathering as much information on these processes as I can.  I plan to put all my findings and results into a separate entry in this blog.

The way this blog is evolving... I am actually working on different projects at the same time.  I will give a chronological account of each project as it evolves.  Rather than jumble the different projects all into one narrative, each project will be discussed within a separate blog entry to which I will add material as it becomes known.  I have this blog entry for woodstove raku.  I will have another one for terrasig.  I still plan to try to develop a shino for electric - will be another topic.  This seems to be a more logical way to present stuff and to keep track of it for myself.

Sunday, February 05, 2012, 8:39:37 PM

Here is a link to my growing list of recipes for raku bodies in the cone 010 range

I have also been researching information on terra sig.

The other day when I put a cone pack into the woodstove, the claybody I used to hold the cones was my cone 10 stoneware with added sand.  I was surprised that the stoneware made it through the firing and it seemed hard and strong  - much denser than the white earthenware body.  So I got the idea that I would make a little pot from this clay ... and put some terra sig on it and test the clay and the sig at the same time.  Well, that didn't work so good.  
  • The pot didn't make it through the firing - blew up
  • The sig wasn't going to work so well because the grog and sand in the clay was poking through, messing up the shine.
I think I had better try out some of my more open clay bodies that were formulated for this firing range.  If I want to do terra sig, I will either have to use a rib to push the grog back into the clay ... or use a layer of slip applied leather hard before I put on the terra sig at the bone dry stage.

I am making up a batch of Jerry Bennett White Terra Sig.  The recipe can be found my link above for "Terra Sig Recipes".  Bennett uses this one as a base for colorants. I think it is interesting and promising. It uses a fairly large amount of non-clay ingredients (talc and zircopax).  If you look at the Peter Pinnell link above, you will see that he reports a tendency toward peeling by white clay terra sigs at specific gravity 1.15 and recommends they always be used with colorants to fix this problem. It's a good bet that the peeling problem results from the sig shrinking too much.... and that any non-shrinking addition would lessen the amount of shrinking.  This is exactly what the Jerry Bennett recipe is going to do. I expect it to be white, to not peel, and to be good with colorants.